In the liquid refining of metals, gas injection is frequently used in order to eliminate impurities, to chemically and thermally homogenize the melt, and to degas and to speed up chemical reactions.
There are principally two means to introduce gases into molten metal through a refractory piece: porous plugs and gas permeable devices provided with slotted refractory plates or with inserts between them. Porous plugs have the ability to introduce small bubbles into the metal in order to improve the gas-metal mass transfer, due to the increased contact area therebetween. Another characteristic of porous plugs is their capacity to avoid metal penetration therebetween when the gas flow is cut off. The main disadvantages of porous plugs are their rapid wear with respect to the surrounding bottom lining and their limited flow capacity.
With gas permeable elements provided with refractory plates having slots or inserts between them (see U.S. Pat. Nos. 4,340,208 and 4,395,026), both the flow capacity and the wear performance have been increased, but with these devices, one cannot introduce small bubbles, and one cannot stop the flow of gas without the risk of metal infiltration between the refractory plates. Such metal penetration is critical in the ladles used for metal treatments because the flow of gas is frequently stopped when liquid metal is in the ladle.
On the other hand, Vayssiere et al. in U.S. Pat. No. 4,340,208 state that the permeability obtained by simply joining the elements together ranges from 4 to 5 liters/second. However, in steelmaking operations such as deep desulphurization or decarburization in large metallurgical vessels, this flow rate is not enough. For example, in order to desulphurize 100 metric tons of steel from 0.03% to 0.003% of sulphur, it is necessary to bubble about 30 liters/second through the molten steel with an appropriate slag.
An object of the present invention is to provide a refractory, gas permeable device which is capable of introducing small bubbles into liquid metals with a wear performance as high as the surrounding lining, without the risk of metal penetration when the gas flow is cut off, and with a wide range of permeability.